1. Field of the Invention
The invention relates generally to transportation vehicles and particularly to a method and apparatus for moving and steering a transit vehicle by controlling the torque transmitted to individual wheels on the vehicle.
2. Description of the Prior Art
There are generally two types of transit vehicles. One type, known as a light rail vehicle, has wheels which rest upon and are guided by a steel rail track which defines the course the vehicle will maintain. The vehicle typically is not suitable for travel on a surface other than the steel track. The second type of transit vehicle, known as a people-mover (bi-directional operating automated guided transit system), uses rubber or steel wheels which rest on the ground to the exclusion of any predirected steel rail tracks. However, this type of vehicle employs a guidance system by which a mechanical linkage in the vehicle steering mechanism physically engages either a guide rail protruding from the roadbed, as described in U.S. Pat. No. 3,515,405 entitled "Axle Suspension System for Transit Vehicles" by N. R. Segar, or the side walls of a trough in which the vehicle rides to steer the vehicle. The propulsion system for such a vehicle is typically a single motor which transmits torque to a differential which then distributes the torque to the driving wheels of the vehicle. Therefore, such a vehicle requires one mechanism for steering and a second mechanism for propelling the vehicle.
Additionally, the steering mechanism for such a vehicle includes a purely mechanical linkage which turns the wheels based upon a force exerted by a mechanical follower in the linkage against guides along a track. This arrangement produces significant lateral forces against the guides. This arrangement requires not only a mechanical linkage that will withstand the forces transmitted from the mechanical follower to steer the wheels but also requires a guide rail with sufficient strength to withstand the lateral forces produced by the mechanical linkage while turning the wheels. Additionally, such a relatively complex mechanical linkage adds weight to the vehicle and therefore requires additional energy to propel the vehicle. Also, the mechanical linkage has physical limitations which restrict the turning radius of the vehicle.
U.S. Pat. No. 3,563,327 teaches a vehicle having a pair of colinear rear axles independently driven to propel and to steer the vehicle. However, the axles are mounted directly to the vehicle frame such that the entire vehicle pivots in the exact fashion of the rear axles. Such an arrangement may be suitable for a vehicle intended to travel in a single direction and permitted to turn about a large radius, since the vehicle will pivot with the axles. However, this arrangement does not permit the vehicle to be bi-directional or to turn about a smaller radius.
It is an object of the present invention to eliminate a significant part of the mechanical linkage used for steering and additionally eliminate the differential as part of a drive train, thereby providing a relatively simple and lightweight mechanical arrangement for propelling and steering a vehicle.
It is a further object of this invention to provide a vehicle with independently driven wheels on a pivoting platform that may be used to turn the vehicle about a small radius and also to propel the vehicle in a forward or rearward direction.